Security Token Missing

I have had a message about a security token being missing whenever i try and reply to a PM. This has sometimes happened when i try to reply to a post but this evening i've tried to reply to two different PMs and am told that a security token is missing so the reply can't go. It prompts you to email the administrator about it but when i try to send the message, it won't go because....you guessed it, the security token is missing.